1. Field of the Invention
The present invention relates to a process for producing vinyl acetate polymers, in particular ethylene-vinyl acetate copolymer, a process for producing high-quality saponified products of vinyl acetate polymers, particularly of ethylene-vinyl acetate copolymer, that are obtained by saponifying the same and cause little coloring and generation of gel-like agglomerates upon molding, and also to resin compositions comprising the saponified products.
2. Description of the Prior Art
Saponified products of vinyl acetate polymers have been used in a variety of applications utilizing their feature of having good hydrophilic property. Among the saponified products, that of ethylene-vinyl acetate is good in melt moldability and especially in oxygen barrier properties, oil resistance, antistatic property, mechanical strength and like properties and hence is useful as packaging material, such as film, sheet or container. The saponified product of ethylene-vinyl acetate copolymer should, when used for various packaging purposes, have the desired properties to meet the intended purposes and, in addition, should not cause even slight coloring of appearance, fish eyes, gel-like agglomerates, coarse grain and poor transparency.
However, the saponified product of ethylene-vinyl acetate copolymer tends to cause, on melt molding, the problems of coloring and generation of gel-like agglomerates. The following various measures have therefore been proposed to suppress the coloring and generation of gel-like agglomerates.
For example, many Japanese Patent Applications disclosing addition of acids such as acetic acid and phosphoric acid or salts thereof have been filed. See Japanese Patent Application Laid-open Nos. 25048/1973, 26993/1976, 28891/1976, 49294/1976, 91988/1976, 954/1977, 955/1977, 956/1977, 20044/1981, 41204/1981, 95043/1986, 95054/1986, 143954/1987 and others. They claim that saponified products of ethylene-vinyl acetate causing little coloring or gel-like agglomerates can be obtained by conducting solution polymerization of ethylene and vinyl acetate with a radical initiator in an organic solvent such as methanol, purging the unreacted vinyl acetate, saponifying the residue by adding an alkali catalyst and, after naturalization and washing, adding on acid such as acetic acid or phosphoric acid, and/or a salt thereof, following by drying.
On the other hand, Japanese Patent Application Laid-open No. 197603/1986 discloses a method of adding a specific aromatic compound after copolymerization of ethylene and vinyl acetate. That is, addition of a specific aromatic compound such as 2,4-diphenyl-4-methyl-1-pentene on completion of copolymerization of ethylene-vinyl acetate copolymer, followed by saponification, can yield a saponified product of the ethylene-vinyl acetate copolymer that generates no irregular odor on melt molding and gives films having good color shade and little fish eye.
As stated above, there have been taken a variety of measures against coloring or generation of gel-like agglomerates of saponified products of vinyl-acetate polymers, in particular that of ethylene-vinyl acetate copolymer.
However, with the addition of an acid such as acetic acid or phosphoric acid and/or a salt thereof, coloring or generation of gel-like agglomerates still occurs if sufficient neutralization and washing after saponification are not conducted or the acid such as acetic acid or phosphoric acid and/or a salt thereof are not added in appropriate amounts. To overcome this drawback, washing is now sufficiently conducted after saponification and the addition amounts of acetic acid or phosphoric acid and a salt thereof have been optimized, only to produce unsatisfactory results.
The addition of a specific aromatic compound (e.g. 2,4-diphenyl-4-methyl-1-pentene) is, as has become clear as a result of a repeating test conducted by the present inventors, can be hardly said to suppress coloring or generation of gel-like agglomerates sufficiently.